Sheeting and chunking has been a problem in commercial polyolefin production reactors for many years. In gas phase reactors, the problem is generally characterized by the formation of solid masses of polymer on the walls of the reactor. These solid masses of polymer (e.g., the sheets) eventually become dislodged from the walls and fall into the reaction section, where they interfere with fluidization, block the product discharge port, plug the distributor plate, and usually force a reactor shut-down for cleaning, any one of which can be termed a “discontinuity event”, which in general is a disruption in the continuous operation of a polymerization reactor. The terms “sheeting, chunking and/or fouling” while used synonymously herein, may describe different manifestations of similar problems, in each case they can lead to a reactor discontinuity event.
There are at least two distinct forms of sheeting that occur in gas phase reactors. The two forms (or types) of sheeting are described as wall sheets or dome sheets, depending on where they are formed in the reactor. Wall sheets are formed on the walls (generally vertical sections) of the reaction section. Dome sheets are formed much higher in the reactor, on the conical section of the dome, or on the hemi-spherical head on the top of the reactor (see, e.g., FIG. 4).
When sheeting occurs with Ziegler-Natta catalysts, it generally occurs in the lower section of the reactor and is referred to as wall sheeting. Ziegler-Natta catalysts are capable of forming dome sheets, but the occurrence is rare. However, with metallocene catalysts, sheeting may occur at either location or both: wall sheeting and dome sheeting.
Dome sheeting has been a particularly troublesome with metallocene catalyst systems. Typical metallocene compounds are generally described as metal complexes containing one or more ligands, usually, cyclopentadienyl derived ligands complexed with a transition metal selected from Group 4, 5 or 6 or from the lanthanide and actinide series of the Periodic Table of Elements.
One characteristic that makes it difficult to control sheeting with metallocene catalysts is their unpredictable static tendencies. For instance, EP 0 811 638 A2 describes metallocene catalysts as exhibiting sudden erratic static charge behavior that can appear after long periods of stable behavior.
As a result of the reactor discontinuity problems associated with using metallocene catalysts, various techniques have been proposed to improve reactor operability. U.S. Pat. Nos. 5,332,706 and 5,473,028 disclose a particular technique for forming a catalyst by “incipient impregnation.” U.S. Pat. Nos. 5,427,991 and 5,643,847 disclose the chemical bonding of non-coordinating anionic activators to supports. U.S. Pat. No. 5,492,975 discloses polymer bound metallocene catalyst systems. U.S. Pat. No. 5,661,095 discloses supporting a metallocene catalyst on a copolymer of an olefin and an unsaturated silane. WO 97/06186 discloses removing inorganic and organic impurities after formation of the metallocene catalyst itself. WO 97/15602 discloses readily supportable metal complexes. WO 97/27224 discloses forming a supported transition metal compound in the presence of an unsaturated organic compound having at least one terminal double bond. U.S. Pat. No. 7,205,363 and WO 2005/003184 disclose the use of certain continuity additives with metallocene catalysts to improve reactor operability.
Others have discussed different process modifications for improving reactor continuity with metallocene catalysts and conventional Ziegler-Natta catalysts. For example, WO 97/14721 discloses the suppression of fines that can cause sheeting by adding an inert hydrocarbon to the reactor. U.S. Pat. No. 5,627,243 discloses a distributor plate for use in fluidized bed gas phase reactors. WO 96/08520 discloses avoiding the introduction of a scavenger into the reactor. U.S. Pat. No. 5,461,123, discloses using sound waves to reduce sheeting. U.S. Pat. No. 5,066,736, and EP-A1 0 549 252, disclose the introduction of an activity retarder to the reactor to reduce agglomerates. U.S. Pat. No. 5,610,244, discloses feeding make-up monomer directly into the reactor above the bed to avoid fouling and improve polymer quality. U.S. Pat. No. 5,126,414, discloses including an oligomer removal system for reducing distributor plate fouling and providing for polymers free of gels. There are various other known methods for improving operability including coating the polymerization equipment, controlling the polymerization rate, particularly on start-up, and reconfiguring the reactor design and injecting various agents into the reactor.
With respect to injecting various agents into the reactor, the use of antistatic agents as process “continuity additives” appear to hold promise and have been the subject of various publications. For example, EP 0 453 116 A1, discloses the introduction of antistatic agents to the reactor for reducing the amount of sheets and agglomerates. U.S. Pat. No. 4,012,574, discloses adding a surface-active compound having a perfluorocarbon group to the reactor to reduce fouling. WO 96/11961, discloses an antistatic agent for reducing fouling and sheeting in a gas, slurry or liquid pool polymerization process as a component of a supported catalyst system. U.S. Pat. Nos. 5,034,480 and 5,034,481, disclose a reaction product of a conventional Ziegler-Natta titanium catalyst with an antistatic agent to produce ultrahigh molecular weight ethylene polymers. For example, WO 97/46599, discloses the use of soluble metallocene catalysts in a gas phase process utilizing soluble metallocene catalysts that are fed into a lean zone in a polymerization reactor to produce stereoregular polymers. WO 97/46599 also discloses that the catalyst feedstream can contain antifoulants or antistatic agents such as ATMER® 163 (commercially available from ICI Specialty Chemicals, Baltimore, Md.). See also U.S. Pat. No. 7,205,363 and WO 2005/003184.
However, adding continuity additives to the reactor has been observed to sometimes result in reduced catalyst productivity.